Apparatus of plural charged-particle beams

ABSTRACT

A multi-beam apparatus for observing a sample with oblique illumination is proposed. In the apparatus, a new source-conversion unit changes a single electron source into a slant virtual multi-source array, a primary projection imaging system projects the array to form plural probe spots on the sample with oblique illumination, and a condenser lens adjusts the currents of the plural probe spots. In the source-conversion unit, the image-forming means not only forms the slant virtual multi-source array, but also compensates the off-axis aberrations of the plurality of probe spots. The apparatus can provide dark-field images and/or bright-field images of the sample.

CLAIM OF PRIORITY

This application is a continuation application of application Ser. No.15/078,369, entitled “Apparatus of Plural Charged-Particle Beams,” filedMar. 23, 2016, which claims the benefit of priority of U.S. provisionalapplication No. 62/137,978, entitled to Ren et al. filed Mar. 25, 2015and entitled “Apparatus of Plural Charged-Particle Beams”, both of whichare incorporated herein by reference in their entireties.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. application Ser. No. 15/065,342entitled to Weiming Ren et al. filed on Mar. 9, 2016 and entitled“Apparatus of Plural Charged-Particle Beams”, the entire disclosures ofwhich are incorporated herein by reference.

This application is related to U.S. application Ser. No. 14/220,358entitled to Zhongwei Chen et al. filed on Mar. 20, 2014 and entitled“Charged Particle Beam Apparatus”, the entire disclosures of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a charged-particle apparatus with aplurality of charged-particle beams. More particularly, it relates to anapparatus which employs plural charged-particle beams to simultaneouslyacquire images of plural scanned regions of an observed area on a samplesurface. Hence, the apparatus can be used to inspect defects and/orparticles on wafers/masks with high detection efficiency and highthroughput in semiconductor manufacturing industry.

2. Description of the Prior Art

For manufacturing semiconductor IC chips, pattern defects and/oruninvited particles (residuals) inevitably appear on surfaces ofwafers/masks during fabrication processes, which reduce the yield to agreat degree. To meet the more and more advanced requirements onperformance of IC chips, the patterns with smaller and smaller criticalfeature dimensions have been adopted. Accordingly, the conventionalyield management tools with optical beam gradually become incompetentdue to diffraction effect, and yield management tools with electron beamare more and more employed. Compared to a photon beam, an electron beamhas a shorter wavelength and thereby possibly offering superior spatialresolution. Currently, the yield management tools with electron beamemploy the principle of scanning electron microscope (SEM) with a singleelectron beam, which therefore can provide higher resolution but can notprovide throughputs competent for mass production. Although the higherand higher beam currents can be used to increase the throughputs, thesuperior spatial resolutions will be fundamentally deteriorated byCoulomb Effect.

For mitigating the limitation on throughput, instead of using a singleelectron beam with a large current, a promising solution is to use aplurality of electron beams each with a small current. The plurality ofelectron beams forms a plurality of probe spots on one being-inspectedor observed surface of a sample. For the sample surface, the pluralityof probe spots can respectively and simultaneously scan a plurality ofsmall scanned regions within a large observed area on the samplesurface. The electrons of each probe spot generate secondary electronsfrom the sample surface where they land on. The secondary electronscomprise slow secondary electrons (energies≤50 eV, simplified as SE forone and SEs for plurality) and backscattered electrons (energies closeto landing energies of the electrons, simplified as BSE for one and BSEsfor plurality). The secondary electrons from the plurality of smallscanned regions can be respectively and simultaneously collected by aplurality of electron detectors. Consequently, the image of the largeobserved area including all of the small scanned regions can be obtainedmuch faster than scanning the large observed area with a single beam.

The plurality of electron beams can be either from a plurality ofelectron sources respectively, or from a single electron source. For theformer, the plurality of electron beams is usually focused onto andscans the plurality of small scanned regions by a plurality of columnsrespectively, and the secondary electrons from each scanned region aredetected by one electron detector inside the corresponding column. Theapparatus therefore is generally called as a multi-column apparatus. Theplural columns can be either independent or share a multi-axis magneticor electromagnetic-compound objective lens (such as U.S. Pat. No.8,294,095). On the sample surface, the beam interval between twoadjacent beams is usually as large as 30-50 mm.

For the latter, a source-conversion unit is used to generate a pluralityof parallel real or virtual images of the single electron source. Eachimage is formed by one part or beamlet of the primary electron beamsgenerated by the single electron source, and therefore can be taken asone sub-source emitting the one beamlet. In this way, the singleelectron source is virtually changed into a plurality of sub-sources ora real or virtual multi-source array. Within the source-conversion unit,the beamlet intervals are at micro meter level so as to make morebeamlets available, and hence the source-conversion unit can be made bysemiconductor manufacturing process or MEMS (Micro Electro-MechanicalSystems) process. Naturally, one primary projection imaging system andone deflection scanning unit within one single column are used toproject the plurality of parallel images onto and scan the plurality ofsmall scanned regions respectively, and one secondary projection imagingsystem focuses the plurality of secondary electron beams therefrom to berespectively detected by a plurality of detection elements of oneelectron detection device inside the single column. The plurality ofdetection elements can be a plurality of electron detectors placed sideby side or a plurality of pixels of one electron detector. The apparatustherefore is generally called as a multi-beam apparatus and theconventional yield management tool with a single electron beam is calledas a single-beam apparatus.

Conventionally, the source-conversion unit comprises one image-formingmeans and one beamlet-forming means or one beamlet-limit means. Theimage-forming means basically comprises a plurality of image-formingelements, and each image-forming element can be a round lens or adeflector. The beamlet-forming means and the beamlet-limit means arerespectively above and below the image-forming means and have aplurality of beam-limit openings. In one source-conversion unit with onebeamlet-forming means, at first the plurality of beam-limit openingsdivides the primary electron beam into a plurality of beamlets, and thenthe plurality of image-forming elements (round lenses or deflectors)focuses or deflects the plurality of beamlets to form the plurality ofparallel real or virtual images. U.S. Pat. Nos. 6,943,349, 7,244,949,7,880,443 respectively propose an multi-beam apparatus with onesource-conversion unit of this type. In one source-conversion unit withone beamlet-limit means, at first the plurality of image-formingelements (deflectors) deflects a plurality of beamlets of the primaryelectron beam to form the plurality of parallel virtual images, and thenthe plurality of beam-limit openings cuts off peripheral electrons ofthe plurality of beamlets respectively. The first cross referenceproposes a multi-beam apparatus with one source-conversion unit of thistype, as shown in FIG. 1.

In FIG. 1, the single electron source 101 on the primary optical axis100_1 generates the primary electron beam 102 seemingly coming from thecrossover 101 s. The condenser lens 110 focuses the primary electronbeam 102 and thereby forming an on-axis virtual image 101 sv of thecrossover 101 s. The peripheral electrons of the primary electron beam102 are cut off by the main opening of the main aperture plate 171. Thesource-conversion unit 120 comprises the micro-deflector array 122 withtwo micro-deflectors 122_2 and 122_3, and a beamlet-limit plate 121 withthree beam-limit openings 121_1, 121_2 and 121_3, wherein the beam-limitopening 121_1 s aligned with the primary optical axis 100_1. If thebeam-limit opening 121_1 is not aligned with the primary optical axis100_1, there will be one more micro-deflector. The micro-deflectors122_2 and 122_3 respectively deflect beamlets 102_2 and 102_3 of theprimary electron beam 102, and thereby forming two off-axis virtualimages 102_2 v and 102_3 v of the crossover 101 s. The deflectedbeamlets 102_2 and 102_3 are perpendicularly incident onto thebeamlet-limit plate 121. The beam-limit openings 121_1, 121_2 and 121_3respectively cut off the peripheral electrons of the center beamlet102_1 of the primary electron beam 102 and the deflected beamlets 102_2and 102_3, and thereby limiting the currents thereof. The focusing powerof the condenser lens no varies the current density of the primaryelectron beam 102, and therefore is able to change the currents of thebeamlets 102_1˜102_3. Consequently, three parallel virtual images 101sv, 102_2 v and 102_3 v form one virtual multi-source array 101 v withvariable currents.

The primary projection imaging system 130 which comprises the transferlens 133 and the objective lens 131, focuses the three beamlets102_1˜102_3 to image the virtual multi-source array low onto thebeing-observed surface 7 and therefore form three probe spots 102_1 s,102_2 s and 102_3 s thereon. The transfer lens 133 focuses the threebeamlets 102_1˜102_3 to perpendicularly land on the being-observedsurface 7. The deflection scanning unit 132 deflects the three beamlets102_1˜102_3 and consequently the three probe spots 102_1 s˜102_3 s scanthree individual regions of the being-observed surface 7. Secondaryelectron beams 102_1 se, 102_2 se and 102_3 se emitted from the threescanned regions are in passing focused by the objective lens 131,deflected by the beam separator 160 to travel along the secondaryoptical axis 150_1, and focused onto and kept within the three detectionelements 140_1, 140_2 and 140_3 of the electron detection device 140respectively by the secondary projection imaging system 150. Thereforeeach detection element will provide an image signal of one correspondingscanned region.

As critical dimensions are shrunk, smaller and smaller particles becomekillers in the yield. For inspecting particles, an electron beam has arelatively shorter wavelength (such as 0.027 nm/2 keV) compared toparticle dimensions (down to several nm), and therefore can providehigher detection sensitivity for small particles than an optical beam.Higher detection efficiency comes from higher detection sensitivity, andhigher detection sensitivity comes from higher image contrast. Theconventional single-beam apparatus scans the being-observed surface withnormal incidence of the primary electron beam, and is criticized indetection sensitivity for particle inspection.

To achieve high detection sensitivity, one dark-field electron-beam(e-beam) inspection method is proposed in the second cross reference,which employs an oblique illumination. As well known, when a primaryelectron beam is incident onto a surface of a sample with an incidenceangle α (relative to the normal of the sample surface), the angulardistribution of the SE emission conforms Lambert's law (proportional tocos ϕ, and ϕ is emission angle relative to the surface normal), and theangular distribution of the BSE emission comprises a diffusely scatteredpart with Lambert's distribution and a reflection-like part withemission maximum in the reflection direction. Obviously, in the casewith an oblique illumination, the SE emission direction and thereflection-like BSE emission direction will be different from eachother. Furthermore, if the sample surface with a particle is obliquelyilluminated by the primary electron beam, due to the difference innormal direction, the sample surface and the particle will be differentin both the SE emission direction and the reflection-like BSE emissiondirection. Please refer to FIG. 8A and FIG. 8B. For the primary electronbeam (102_1), the SEs (102_1 b 1) and BSEs (102_1 b 2) from the samplesurface (7) display a regular scattering situation and therefore becomesignal electrons for a bright-field image, while the SEs (102_1 d 1) andBSEs (102_1 d 2) from the particle (7_P) displays an irregularscattering situation and therefore become signal electrons for adark-field image. The dark-field e-beam method employs the differencebetween the regular scattering on a sample surface and the irregularscattering on a particle thereon. A dark-field BSE imaging, which has ahigh image contrast due to the particle, can be obtained by specificallyarranging oblique illumination, collection of BSEs and guiding SEs.

Accordingly, it is necessary to provide a multi-beam which cansimultaneously obtain images of plural subareas of an area on a samplesurface with high image contrast and high throughput. Especially, amulti-beam apparatus which can detect uninvited particles onwafers/masks with high detection sensitivity and high throughput isneeded to match the roadmap of the semiconductor manufacturing industry.

SUMMARY OF THE INVENTION

The object of this invention is to provide a multi-beam apparatus withan oblique illumination on a being-observed surface of a sample. Insemiconductor manufacturing industry, the apparatus can function as ayield management tool to inspect defects and/or particles onwafers/masks with high detection efficiency and high throughput. Thesample surface is slant to the primary optical axis of the apparatus.The apparatus employs a new source-conversion unit to form a slantvirtual multi-source array with a plurality of beamlets from a singleelectron source, a primary projection imaging system to project themulti-source array onto the sample surface and therefore form aplurality of probe spots thereon with oblique illuminations of theplurality of beamlets. In the new source-conversion unit, eachimage-forming element comprises a deflector and a round-lens, andtherefore the multi-source array can be tilted to match the obliqueillumination. Each image-forming element may further comprise astigmator to compensate the astigmatism aberration of the correspondingprobe spot. The apparatus may further comprise a condenser lens toadjust the currents of the plurality of probe spots.

The apparatus may use a beam separator to separate a plurality ofdark-field signal electron beams and the plurality of beamlets, adark-field secondary projection imaging system and a dark-field electrondetection device with a plurality of dark-field detection elements tofocus and detect the plurality of dark-field signal electron beams, andtherefore obtain a plurality of dark-field images, wherein animage-contrast-enhancing electrode may be employed to increase theirimage contrasts. The apparatus may use a bright-field secondaryprojection imaging system and a bright-field electron detection devicewith a plurality of bright-field detection elements to focus and detecta plurality of bright-field signal electron beams, and therefore obtaina plurality of bright-field images. The apparatus can operate in amulti-beam mode or a single-beam mode, and a single-beam detector may beadded to make the single-beam mode easy to use.

Accordingly, the invention therefore provides a multi-beam apparatus forobserving a surface of a sample, which comprises an electron source, acondenser lens below the electron source, a source-conversion unit belowthe condenser lens, a primary projection imaging system below thesource-conversion unit and comprising an objective lens and a transferlens, a deflection scanning unit inside the primary projection imagingsystem, a sample stage below the primary projection imaging system, abeam separator above the objective lens, a dark-field secondaryprojection imaging system above the beam separator, and a dark-fieldelectron detection device with a plurality of dark-field detectionelements. The source-conversion unit comprises an image-forming meanswith a plurality of image-forming elements and a beamlet-limit meanswith a plurality of beam-limit openings. The image-forming means isabove the beamlet-limit means, and each image-forming element comprisesa micro-deflector and a micro-round-lens. The electron source, thecondenser lens, the source-conversion unit, the primary projectionimaging system, the deflection scanning unit and the beam separator arealigned with a primary optical axis of the apparatus. The sample stagesustains the sample so that the surface faces to the objective lens andis slant to the primary optical axis. The dark-field secondaryprojection imaging system and the dark-field electron detection deviceare aligned with a dark-field secondary optical axis of the apparatus,and the dark-field secondary optical axis is not parallel to the primaryoptical axis. The electron source generates a primary electron beamalong the primary optical axis. A plurality of micro-deflectors of theplurality of image-forming elements deflects a plurality of beamlets ofthe primary electron beam to form a plurality of parallel virtual imagesof the electron source and therefore a virtual multi-source array isconverted from the electron source. A plurality of micro-round-lenses ofthe plurality of image-forming elements respectively focuses theplurality of beamlets to tilt the virtual multi-source array slant tothe primary optical axis. The plurality of beamlets passes through theplurality of beam-limit openings respectively. A current of each beamletis therefore limited by one corresponding beam-limit opening, andcurrents of the plurality of beamlets can be varied by adjusting thecondenser lens. The primary projection imaging system images the virtualmulti-source array onto the slant surface. The virtual multi-sourcearray is tilted to make an image plane of the primary projection imagingsystem coincident with the slant surface. A plurality of probe spots istherefore formed thereon, and the deflection scanning unit deflects theplurality of beamlets to scan the plurality of probe spots respectivelyover a plurality of scanned regions within an observed area on thesurface. A plurality of dark-field signal electron beams generated bythe plurality of probe spots respectively from the plurality of scannedregions is in passing focused by the objective lens, deflected by thebeam separator to the dark-field secondary projection imaging system,focused onto and kept within the plurality of dark-field detectionelements by the dark-field secondary projection imaging system, andtherefore the plurality of dark-field detection elements respectivelyprovides a plurality of dark-field images with respect to the pluralityof scanned regions.

The multi-beam apparatus may further comprise a main aperture platebelow the electron source, which has a main opening aligned with theprimary optical axis and functions as a beam-limit aperture for theprimary electron beam. The transfer lens may focus the plurality ofbeamlets to land on the surface with equal amount of incidence angles.That each image-forming element may comprise a micro-stigmator forcompensating astigmatism aberration of one corresponding probe spot. Theincidence angles can be varied by the deflection scanning unit, and canbe varied by the plurality of micro-deflectors. The apparatus canoperate a single-beam mode. The multi-beam apparatus may furthercomprise a single-beam electron detector above the beam separator, whichcan be used in the single-beam mode. The each image-forming element mayhave a 4-pole structure which can function as the micro-deflector andthe micro-round-lens.

The multi-beam apparatus may further comprise animage-contrast-enhancing electrode, which is placed above the surface toattract SEs generated by the plurality of beamlets not detected by theplurality of dark-field detection elements and therefore enhances imagecontrasts of the plurality of dark-field images. The multi-beamapparatus may further comprise a bright-field secondary projectionimaging system and a bright-field electron detection device with aplurality of bright-field detection elements, wherein both are placedabove the surface, along and aligned with a bright-field secondaryoptical axis not parallel to the primary optical axis. A plurality ofbright-field signal electron beams generated by the plurality of probespots respectively from the plurality of scanned regions can be focusedonto and kept within the plurality of bright-field detection elements bythe bright-field secondary projection imaging system, and therefore theplurality of bright-field detection elements respectively may provide aplurality of bright-field images with respect to the plurality ofscanned regions.

The present invention also provides a multi-beam apparatus for observinga surface of a sample, which comprises an electron source, a condenserlens below the electron source, a source-conversion unit below thecondenser lens, a primary projection imaging system below thesource-conversion unit and comprising an objective lens and a transferlens, a deflection scanning unit inside the primary projection imagingsystem, a sample stage below the primary projection imaging system, abright-field secondary projection imaging system above the sample stage,and a bright-field electron detection device with a plurality ofbright-field detection elements. The source-conversion unit comprises animage-forming means with a plurality of image-forming elements and abeamlet-limit means with a plurality of beam-limit openings. Theimage-forming means is above the beamlet-limit means, and eachimage-forming element comprises a micro-deflector and amicro-round-lens. The electron source, the condenser lens, thesource-conversion unit, the primary projection imaging system, and thedeflection scanning unit are aligned with a primary optical axis of theapparatus. The sample stage sustains the sample so that the surfacefaces to the objective lens and is slant to the primary optical axis.The bright-field secondary projection imaging system and thebright-field electron detection device are aligned with a bright-fieldsecondary optical axis of the apparatus, and the bright-field secondaryoptical axis is not parallel to the primary optical axis. The electronsource generates a primary electron beam along the primary optical axis.A plurality of micro-deflectors of the plurality of image-formingelements deflects a plurality of beamlets of the primary electron beamto form a plurality of parallel virtual images of the electron sourceand therefore a virtual multi-source array is converted from theelectron source. A plurality of micro-round-lenses of the plurality ofimage-forming elements respectively focuses the plurality of beamlets totilt the virtual multi-source array slant to the primary optical axis.The plurality of beamlets passes through the plurality of beam-limitopenings respectively. A current of each beamlet is therefore limited byone corresponding beam-limit opening, and currents of the plurality ofbeamlets can be varied by adjusting the condenser lens. The primaryprojection imaging system images the virtual multi-source array onto theslant surface. The virtual multi-source array is tilted to make an imageplane of the primary projection imaging system coincident with the slantsurface. A plurality of probe spots is therefore formed thereon, and thedeflection scanning unit deflects the plurality of beamlets to scan theplurality of probe spots respectively over a plurality of scannedregions within an observed area on the surface. A plurality ofbright-field signal electron beams generated by the plurality of probespots respectively from the plurality of scanned regions is focused ontoand kept within the plurality of bright-field detection elements by thebright-field secondary projection imaging system, and therefore theplurality of bright-field detection elements respectively provides aplurality of bright-field images with respect to the plurality ofscanned regions.

The multi-beam apparatus may further comprise a main aperture platebelow the electron source, which has a main opening aligned with theprimary optical axis and functions as a beam-limit aperture for theprimary electron beam. The transfer lens may focuse the plurality ofbeamlets to land on the surface with equal amount of incidence angles.That each image-forming element may comprise a micro-stigmator forcompensating astigmatism aberration of one corresponding probe spot. Theincidence angles can be varied together by the deflection scanning unitand can be varied by the plurality of micro-deflectors. That eachimage-forming element may have a 4-pole structure which can function asthe micro-deflector and the micro-round-lens. That each image-formingelement may have an 8-pole structure which can function as themicro-deflector, the micro-round-lens and the micro-stigmator. That eachimage-forming element may comprise an upper 4-pole structure and a lower4-pole structure thereunder, and the upper 4-pole structure and thelower 4-pole structure can be aligned with each other and have a 45°difference in azimuth. The upper 4-pole structure and the lower 4-polestructure may function as the micro-deflector, the micro-round-lens andthe micro-stigmator.

The present invention also provides a method for observing a surface ofa specimen, which comprises steps of providing a single charged particlebeam, forming a virtual source array being slant to the surface and froma plurality of beamlets of the single charged particle beam, projectingthe virtual source array onto the surface with the plurality of beamletsobliquely landing on the surface, and receiving a plurality of signalcharged particle beams emanated from the surface.

The plurality of signal charged particle beams may comprise a pluralityof dark-field signal electron beams, or a plurality of bright-fieldsignal electron beams.

The present invention also provides an apparatus of plural chargedparticle beam, which comprises a single charged particle source foremitting a single charged particle beam, a source-conversion unit forforming a virtual source array being slant to a surface being observedand from a plurality of beamlets of the single charged particle beam, aprimary projection imaging system for projecting the virtual sourcearray onto the surface with the plurality of beamlets obliquely landingon the surface, a secondary projection imaging system for influencing aplurality of signal charged particle beams emanated from the surface,and a detection device for receiving the plurality of signal chargedparticle beams influenced by the secondary projection imaging system.

The apparatus may further comprise means for adjusting beam currentbetween the single charged particle source and the source-conversionunit. The apparatus may further comprise a deflection scanning unit,above the surface, for scanning the plurality of beamlets on thesurface. The apparatus may further comprise a beam separator, above anobjective lens of the primary projection imaging system, for guiding aplurality of dark-field signal electron beams of the plurality of signalcharged particle beams to the secondary projection imaging system.

Other advantages of the present invention will become apparent from thefollowing description taken in conjunction with the accompanyingdrawings wherein are set forth, by way of illustration and example,certain embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings,wherein like reference numerals designate like structural elements, andin which:

FIG. 1 is a schematic illustration of one configuration of aconventional multi-beam apparatus.

FIG. 2A is a schematic illustration of one configuration of a newmulti-beam apparatus in accordance with one embodiment of the presentinvention.

FIGS. 2B and 2C are respectively a schematic illustration of oneoperation mode of the new multi-beam apparatus in FIG. 2A.

FIG. 3 is a schematic illustration of another configuration of the newmulti-beam apparatus in accordance with another embodiment of thepresent invention.

FIGS. 4A˜4E are respectively a schematic illustration of a configurationof an image-forming means in FIG. 2A in accordance with anotherembodiment of the present invention.

FIGS. 5A and 5B are respectively a schematic illustration of aconfiguration of an image-forming means in FIG. 3 in accordance withanother embodiment of the present invention.

FIGS. 6A and 6B are schematic illustrations of a configuration of animage-forming means in FIG. 3 in accordance with another embodiment ofthe present invention.

FIG. 7 is a schematic illustration of another configuration of the newmulti-beam apparatus and one operation mode thereof in accordance withanother embodiment of the present invention.

FIGS. 8A˜8C are schematic illustrations on the function of theimage-contrast-enhancing electrode in the new multi-beam apparatus inFIG. 7.

FIG. 9 is a schematic illustration of another configuration of the newmulti-beam apparatus and one operation mode thereof in accordance withanother embodiment of the present invention

FIG. 10 is a schematic illustration of another configuration of the newmulti-beam apparatus and one operation mode thereof in accordance withanother embodiment of the present invention.

FIG. 11 is a schematic illustration of one operation mode of the newmulti-beam apparatus in FIG. 3.

FIG. 11B is a schematic illustration of another configuration of the newmulti-beam apparatus and one operation mode thereof in accordance withanother embodiment of the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Various example embodiments of the present invention will now bedescribed more fully with reference to the accompanying drawings inwhich some example embodiments of the invention are shown. Withoutlimiting the scope of the protection of the present invention, all thedescription and drawings of the embodiments will exemplarily be referredto an electron beam. However, the embodiments are not be used to limitthe present invention to specific charged particles.

In the drawings, relative dimensions of each component and among everycomponent may be exaggerated for clarity. Within the followingdescription of the drawings, the same or like reference numbers refer tothe same or like components or entities, and only the differences withrespect to the individual embodiments are described. For sake ofclarity, only three beamlets are available in the drawings, but thenumber of beamlets can be anyone.

Accordingly, while example embodiments of the invention are capable ofvarious modifications and alternative forms, embodiments thereof areshown by way of example in the drawings and will herein be described indetail. It should be understood, however, that there is no intent tolimit example embodiments of the invention to the particular formsdisclosed, but on the contrary, example embodiments of the invention areto cover all modifications, equivalents, and alternatives falling withinthe scope of the invention.

In this invention, “axial” means “in the optical axis direction of alens (round or multi-pole), an imaging system or an apparatus”, “radial”means “in a direction perpendicular to the optical axis”, “on-axial”means “on or aligned with the optical axis” and “off-axis” means “not onor not aligned with the optical axis”.

In this invention, “primary electrons” means “electrons emitted from anelectron source and incident onto a being-observed or inspected surfaceof a sample, and “secondary electrons” means “electrons generated fromthe surface by the “primary electrons”.

In this invention, “bright-field signal electrons” means “secondaryelectrons generated from a being-observed or inspected surface of asample by a primary electron beam”, and “dark-field signal electrons”means “secondary electrons generated from a particle on the surface bythe primary electron beam”.

In this invention, X, Y and Z axe form Cartesian coordinate. The primaryoptical axis of a multi-beam apparatus is on the Z-axis, and the beam ofprimary electrons travels along the Z-axis.

In this invention, “an illumination angle” means “the smaller anglebetween the primary optical axis of a multi-beam apparatus and thenormal of the being-observed surface of a sample therein”, “normalillumination” means “the illumination angle is zero” and “an obliqueillumination” means “the illumination angle is not zero”.

In this invention, “single-beam mode” means only one beamlet is in use,and “multi-beam mode” means a least two beamlets are in use.

In this invention, all terms relate to through-holes, openings andorifices mean openings or holes penetrated through one plate.

Next, the present invention will provide some embodiments of a newmulti-beam apparatus for observing a sample surface with an obliqueillumination. The sample surface is slant to the primary optical axis ofthis multi-beam apparatus, and forma a large illumination angle (such as30° or larger). The multi-beam apparatus employs a new source-conversionunit for generating a plurality of parallel virtual images with aplurality of beamlets from a single electron source so as to form avirtual multi-source array which is slant to the primary optical axis, aprimary projection imaging system for imaging the virtual multi-sourcearray onto the sample surface so as to form a plurality of probe spotsthereon. The multi-beam apparatus may also use a condenser lens foradjusting the currents of the plurality of probe spots. In the newsource-conversion unit, each image-forming element comprises a deflectorand a round-lens, and therefore the virtual multi-source array can betilted to match the slant sample surface. Each image-forming element mayalso comprise a stigmator to compensate astigmatism aberration of thecorresponding probe spot.

The multi-beam apparatus uses a beam separator for guiding a pluralityof dark-field signal electron beams, and a dark-field secondaryprojection imaging system for focusing and keeping the plurality ofdark-field signal electron beams onto a plurality of dark-fielddetection elements of a dark-field electron detection device. Thereforea plurality of dark-field images can be obtained by the plurality ofdark-field detection elements. In this case, a contrast-enhancingelectrode can be used to prevent a plurality of bright-field signalelectron beams to be detected by the plurality of dark-field detectionelements and hence enhance the image contrasts of the plurality ofdark-field images. The multi-beam apparatus can also use a bright-fieldsecondary projection imaging system to focus and keep the plurality ofbright-field signal electron beams onto a plurality of bright-fielddetection elements of a bright-field electron detection device.Therefore a plurality of bright-field images can be obtained by theplurality of bright-field detection elements.

One embodiment 200A of the new mutli-beam apparatus is shown in FIG. 2A.The being-observed surface 7 of the sample 8 will be supported by astage (not shown here) to form an illumination angle α. The singleelectron source 101 is on the primary optical axis 200_1. The condenserlens 110, the main aperture plate 171, the new source-conversion unit120-1, the primary projection imaging system 13 o, the deflectionscanning unit 132 and the beam separator 160 are placed along andaligned with the primary optical axis 200_1. The dark-field secondaryprojection imaging system 150 and the dark-field electron detectiondevice 140 are placed along and aligned with the dark-field secondaryoptical axis 150_1.

The main aperture plate 171 can be placed above the condenser lens nofor reducing Coulomb Effect as early as possible, or immediately abovethe new source-conversion unit 120-1 as shown here. In the newsource-conversion unit 120-1, the image-forming means is themicro-deflector-lens array 122-1 with three micro-deflector-lenselements 122_1DL, 122_2DL and 122_3DL, and the beamlet-limit means isthe beamlet-limit plate 121 with three beam-limit openings 121_1, 121_2and 121_3. Each micro-deflector-lens element is one image-formingelement, and comprises a micro-deflector and a micro-round-lens. If thebeam-limit opening 121_1 is aligned with the primary optical axis 200_1as shown here, the micro-deflector-lens element 122_1DL can onlycomprise one micro-round-lens. The primary projection imaging system 130comprises a transfer lens 133 and an objective lens 131. The deflectionscanning unit 132 comprises at least one deflector. The beam separator160 comprises a Wien Filter. The dark-field electron detection device140 comprises three dark-field detection elements 140_1, 140_2 and140_3.

FIGS. 2B and 2C show two operation modes of the new multi-beam apparatus200A. The single electron source 101 comprises a cathode, an extractionand/or an anode, wherein the primary electrons are emitted from thecathode and extracted and/or accelerated to form a primary electron beam102 with high energy (such as 8˜20 keV), a high angular intensity (suchas 0.5˜5 mA/sr) and a crossover (virtual or real) 101 s shown by theon-axis oval mark here. Therefore it is convenient to think that theprimary electron beam 102 is emitted from the crossover 101 s, and thesingle electron source 101 is simplified to be the crossover

In FIG. 2B, the condenser lens 110 is off. The primary electron beam 102passes through the condenser lens 110 without focusing influence and itsperipheral electrons are cut off by the main opening of the mainaperture plate 171. The micro-deflectors of the micro-deflector-lenselements 122_2DL and 122_3DL respectively deflect beamlets 102_2 and102_3 of the primary electron beam 102. The deflected beamlets 102_2 and102_3 respectively form the off-axis virtual images 102_2 v and 102_3 vof the crossover 101 s. The micro-round-lenses of themicro-deflector-lens elements 122_2DL and 122_3DL respectively focusbeamlets 102_2 and 102_3 to move the virtual images 102_2 v and 102_3 vupwards, and the micro-round-lens of the micro-deflector-lens element122_1DL focuses the on-axis beamlet 102_1 of the primary electron beam102 to form an on-axis virtual image 102_1V of the crossover 101 s. Thevirtual images 102_1 v˜102_3 v form the virtual multi-source array 101 vwhich is slant to the primary optical axis 200_1. The deflected beamlets102_2 and 102_3 are parallel or substantially parallel to the primaryoptical axis 100_1 and therefore perpendicularly incident onto thebeamlet-limit plate 121. The beam-limit openings 121_1˜121_3respectively cut off the peripheral electrons of the beamlets102_1˜102_3, and thereby limiting the currents thereof.

Next the virtual images 102_1 v-102_3 v are imaged onto the surface 7 bythe transfer lens 133 and the objective lens 131, and form three probespots 102_1 s, 102_2 s and 102_3 s thereon. The micro-round-lenses ofthe micro-deflector-lens elements 122_1DL˜122_3DL adjust the virtualmulti-source array 101 v to make the image plane of the primaryprojection imaging system 130 coincident with the slant sample surface7. The transfer lens 133 focuses the three besmlets 102_1˜102_3 to landon the surface 7 with same angles of incidence. Hence the three besmlets102_1˜102_3 will substantially pass through the front focal point of theobjective lens 131. If the objective lens 131 comprises one magneticlens, the two off-axis beamlets 102_2 and 102_3 may not exactly passthrough the front focal point due to the influence of magnetic rotation,and this is very helpful to reduce the Coulomb Effect at the beamletcrossover CS. The deflection scanning unit 132 deflects the threebeamlets 102_1˜102_3 and consequently the three probe spots 102_1S˜102_3 s scan three individual regions on the surface 7.

The dark-field signal electron beams 102_1 d, 102_2 d and 102_3 demitted from the three scanned regions at first are in passing focusedand deflected by the objective lens 131 and the deflection scanning unit132 respectively. Then the dark-field signal electron beams 102_1 d,102_2 d and 102_3 d are deflected to travel along the dark-fieldsecondary optical axis 150_1 by the beam separator 160, and respectivelyfocused onto the dark-field detection elements 140_1, 140_2 and 140_3 ofthe dark-field electron detection device 140 by the dark-field secondaryprojection imaging system 150. Therefore each detection element willprovide an dark-field image signal of one corresponding scanned region.

To keep the focused dark-field signal electron beams 102_1 d˜102_3 dwithin the dark-field detection elements 140_1˜140_3 so as to avoidcross-talks therebetween, the dark-field secondary projection imagingsystem 150 may comprise one anti-scanning deflector to compensate thedeflection influence of deflection scanning unit 132. Different samplesusually request different observing conditions, such as the landingenergies and the currents of the beamlets 102_1˜102_3. This isespecially true for inspecting wafers/masks in semiconductormanufacturing industry. The focusing power of the objective lens 131will be changed with respect to the observing conditions, and theinfluence on the dark-field signal electron beams 102_1 d-102_3 d willvary accordingly. Therefore the dark-field secondary projection imagingsystem 150 may comprise one zoom lens to compensate the variation of theobjective lens 131 in focusing influence, and one anti-rotation magneticlens to compensate the variation in rotation influence if the objectivelens 131 comprises a magnetic lens. If the dark-field electron detectiondevice 140 is perpendicular to the dark-field secondary optical axis150_1, the slant surface 7 will defocus the dark-field signal electronbeams 102_1 d˜102_3 d, and therefore enlarge the sizes thereof. Thedefocusing may make a part of one focused dark-field signal electronbeam out of the corresponding dark-field detection element and therebygenerating the cross-talks too. Hence the dark-field electron detectiondevice 140 can be tilted to compensate the defocusing due to the slantsurface 7.

Each of the two off-axis probe spots 102_2 s and 102_3 s comprises theoff-axis aberrations generated by the objective lens 131, the transferlens 133 and the condenser lens no when being turned on. The off-axisaberrations of each off-axis probe spot can be reduced by individuallyoptimizing the trajectory of the corresponding beamlet. The static partsof the off-axis aberrations can be reduced by adjusting the deflectionpower of the corresponding micro-deflector. The dynamic parts of theoff-axis aberrations can be reduced by optimizing the performance of thedeflection scanning unit 132 which therefore may comprise more than onedeflector for realizing scanning with smaller aberrations. In addition,the left field curvature aberrations of the two off-axis probe spots102_2 s and 102_3 s can be compensated by adjusting the focusing powersof the micro-round-lenses of the micro-deflector-lens elements 122_2DLand 122_3DL.

The condenser lens no is turned on in FIG. 2C, which focuses the primaryelectron beam 102 and therefore moves the virtual multi-source array 101v further upward. The focusing function of the condenser lens noincrease the current density of the focused primary electron beam 102,and thereby increasing the currents of the beamlets 102_1˜102_3 higherthan in FIG. 2B. Hence, the currents of all the beamlets can becontinuously adjusted by the condenser lens 110.

Similar to a conventional SEM, the size of each probe spot is minimizedby balancing the geometric and diffraction aberrations, Gaussian imagesize and Coulomb effect. The focusing function of the condenser lens 110changes the imaging magnification from the crossover 101 s to thebeing-observed surface 7, which influences the balance and therefore mayincrease the size of each probe spot. To avoid largely increasing thesizes of the probe spots when the currents of the beamlets are largelyvaried, the sizes of the beam-limit openings 121_1˜121_3 can beaccordingly changed. Consequently, the beamlet-limit plate 121 ispreferred having multiple groups of beam-limit openings. The sizes ofbeam-limit openings in a group are different from those in anothergroup. Alternately, the focusing power of the transfer lens 133 can bechanged to reduce the variation of the imaging magnification. Thetrajectories of the off-axis beamlets 102_2 and 102_3 will be influencedby the focusing power variation of the transfer lens 133, and deflectionpowers of the micro-deflectors of the micro-deflector-lens elements122_2DL and 122_3DL can be accordingly adjusted to keep thetrajectories. In this way, the beamlets 102_2 and 102_3 may be slightlynot parallel to the primary optical axis 200_1.

In FIG. 2B and FIG. 2C, the beamlets 102_1˜102_3 land on the surface 7along the primary optical axis 200_1, i.e. their angles of incidence areequal to the illumination angle α. The beamlets 102_1˜102_3 can also betilted to land on the surface 7 with same angles of incidence but notequal to the illumination angle α. The emission direction of eachdark-field signal electron beam varies with the angle of incident of thecorresponding beamlet, and therefore the tilting the correspondingbeamlet may improve the image contrast. The beamlets 102_1˜102_3 can betilted by the deflection scanning unit 132, or by the micro-deflectorsof the micro-deflector-lens elements 122_1D˜122_3DL.

Another embodiment 210A of the new multi-beam apparatus is shown in FIG.3, which uses another new source-conversion unit 120-2. The newsource-conversion unit 120-2 is different from the new source-conversionunit 120-1 in the image-forming means, wherein the image-forming meansis the micro-deflector-lens-and-compensator array 122-2 with threemicro-deflector-lens-and-compensator elements 122_1 dc, 122_2 dc and122_3 dc. Each micro-deflector-lens-and-compensator element is oneimage-forming element, and comprises a micro-deflector, amicro-round-lens and a micro-stigmator. Same as the micro-deflector-lensarray 122-1 in FIG. 2A, the micro-deflectors and the micro-round-lensesare used to change the single electron source 101 into one slant virtualmulti-source array which makes the image plane of the primary projectionimaging system 130 coincide with the slant being-observed surface 7 ofthe sample 8, and the micro-round-lenses are used to compensate thefield curvature aberrations of the images of the slant multi-sourcearray on the surface 7. With the new source-conversion unit 120-2, themicro-stigmators can compensate the astigmatism aberrations of theimages and thereby further improving image resolutions. In comparisonwith the micro-deflector-lens array 122-1, themicro-deflector-lens-and-compensator array 122-2 is an advancedimage-forming means.

Each of the micro-deflector-lens elements 122_1DL˜122_3DL in FIG. 2A cansimply comprise two curved electrodes as shown in FIG. 4A. To clearexpress how to orientate two curved electrodes of eachmicro-deflector-lens element, three more 122_11DL, 122_21DL and 122_31DLare shown. Within each micro-deflector-lens element, the inner surfacesof the two curved electrodes form a circular shape and are perpendicularto the required deflection direction of the corresponding beamlet.Therefore the two curved electrodes can be set at voltages to generateone dipole field (i.e. deflection field) in the required deflectiondirection and one round-lens field. For example, themicro-deflector-lens element 122_2DL has two curved electrodesperpendicular to the X-axis and therefore can deflect the beamlet 102_2in the X-axis direction, and the micro-deflector-lens element 122_11DLhas two curved electrodes perpendicular to the Y-axis and therefore candeflect one beamlet in the Y-axis direction. The on-axismicro-deflector-lens element 122_1DL is only required to focus theon-axis beamlet 102_1, and therefore can be formed by one annularelectrode for generating one round-lens field.

From the manufacturing point of view, all the micro-deflector-lenselements are at least preferred to have same configurations.Accordingly, the micro-deflector-lens element 122_1DL can have the sameconfiguration as others, such as the micro-deflector-lens element122_2DL as shown in FIG. 4B. Due to all the micro-deflector-lenselements are not same in orientation, it is difficult to make onemicro-deflector-lens array 122-1 comprising a large number of themicro-deflector-lens elements. From the manufacturing point of view, allthe elements are preferred to have same configuration and sameorientation in geometry. Hence a micro-deflector-lens element with aquadrupole or 4-pole configuration can meet this requirement, as shownin FIG. 4C. The inner surfaces of the four curved electrodes of eachmicro-deflector-lens element form a circular shape, and therefore adeflection field in any direction and one round-lens field can thereforebe generated.

To operate one micro-deflector-lens element, a driving-circuit needsconnecting with each electrode thereof. In FIG. 4D, to prevent thedriving-circuits from being damaged by the primary electron beam 102,one upper electric-conduction plate 122-CL1 with three upperthrough-holes is placed above the electrodes of all themicro-deflector-lens elements 122_1DL˜122_3DL. The upperelectric-conduction plate 122-CL1 also shortens the upper fringe rangesof the fields of all the micro-deflector-lens elements. The lowerelectric-conduction plate 122-CL2 with three lower through-holesshortens the lower fringe ranges of the fields of all themicro-deflector-lens elements. The upper insulator plates 122-IL1 withthree upper orifices and the lower insulator plates 122-IL2 with threelower orifices support the micro-deflector-lens elements 122_1DL˜122_3DLand therefore make the image-forming means 122-1 more stable inconfiguration.

The upper through-holes, the upper orifices, the lower through-holes andthe lower orifices are aligned with the micro-deflector-lens elementsrespectively. For each micro-deflector-lens element, the radial sizes ofthe corresponding upper and lower through-holes are equal to or smallerthan the inner radial dimensions of the curved electrodes, and smallerthan the radial sizes of the corresponding upper and lower orifices. Toreduce the electrons scattered from the sidewall of each upperthrough-hole, each upper through-hole is preferred in an upside-downfunnel shape; i.e. the small end thereof is on the entrance side, asshown in FIG. 4E.

Each of the micro-deflector-lens-and-compensator elements 122_1 dc˜122_3dc in FIG. 3 can simply have a quadrupole or 4-pole configuration, asshown in FIG. 5A. To clear express how to orientate four curvedelectrodes of each micro-deflector-lens element, three more 122_11 dc,122_21 dc and 122_31 dc are shown. Within eachmicro-deflector-lens-and-compensator element, the inner surfaces of thefour curved electrodes form a circular shape and two of the four curvedelectrodes are perpendicular to the required deflection direction of thecorresponding beamlet. Therefore the four curved electrodes can be setat voltages to generate one round-lens field, one dipole field (i.e.deflection field) in any direction and one quadrupole field in therequired deflection direction. For example, themicro-deflector-lens-and-compensator element 122_2 dc has two curvedelectrodes perpendicular to the X-axis and therefore can generate onequadrupole field in the X-axis direction, and themicro-deflector-lens-and-compensator element 122_31 dc has two curvedelectrodes perpendicular to the direction 122_31 dc_2 and therefore cangenerate one quadrupole field in this direction. The on-axismicro-deflector-lens-and-compensator element 122_1 dc is only requiredto focus the on-axis beamlet 102_1, and therefore can be orientated inany directions.

From the manufacturing point of view, all themicro-deflector-lens-and-compensator elements are most preferred to havesame configurations and same orientation in geometry. An octupole-lensstructure comprising eight curved electrodes whose inner surfaces form acircular shape, can generate one deflection field in any direction, oneround-lens field and one quadrupole field in any direction. Thereforeall the micro-deflector-lens-and-compensator elements can have sameoctupole-lens structures oriented in same directions, as shown in FIG.5B. For the same reasons mentioned above, themicro-deflector-lens-and-compensator elements can be sandwiched by theupper and lower electric-conduction plates 122-CL1 and 122-CL2 and theupper and lower insulator plates 122-IL1 and IL2 in the same way as theimage-forming means 122-1 in FIG. 4E.

FIG. 6A shows another embodiment of themicro-deflector-lens-and-compensator array 122-2 in FIG. 3. Eachmicro-deflector-lens-and-compensator element comprises a pair of 4-poleconfigurations which are placed in two layers, aligned with each otherand have a 45° difference in azimuth or orientation. Themicro-deflector-lens-and-compensator element 122_1 dc, 122_2 dc and122_3 dc respectively comprise the pair of the upper and lower 4-poleconfigurations 122_1 dc-1 and 122_1 dc-2, the pair of the upper andlower 4-pole configurations 122_2 dc-1 and 122_2 dc-2, and the pair ofthe upper and lower 4-pole configurations 122_3 dc-1 and 122_3 dc-2. Theupper 4-pole configurations 122_1 dc-1, 122_2 dc-1 and 122_3 dc-1 areplaced in one upper layer. The lower 4-pole configurations 122_1 dc-2,122_2 dc-2 and 122_3 dc-2 are placed in one lower layer, respectivelybelow and aligned with the upper 4-pole configurations 122_1 dc-1, 122_2dc-1 and 122_3 dc-1. For example, with respect to the X axis, theazimuths of the upper and lower 4-pole configurations 122_1 dc-2C-1 and122_2 dc-2 are respectively 0° and 45° as shown in FIG. 6B. For the samereasons mentioned above, the upper and lower layers are shielded by theupper and lower electric-conduction plate 122-CL1 and 122-CL2, andsupported by the upper and lower insulator plates 122-IL1 and 122-IL2and a middle insulator plate 122-IL3 with multiple middle orifices. Foreach micro-deflector-lens-and-compensator element, the deflection fieldin any desired direction and the round-lens field can be generated byeither or both of the upper and lower 4-pole configurations, and thequadrupole field in any direction can be generated by both of the upperand lower 4-pole configurations.

Another embodiment 220A of the new multi-beam apparatus and its oneoperation mode are shown in FIG. 7, which uses animage-contrast-enhancing electrode 143 to enhance the image contrasts ofthe images detected by the dark-field electron detection device 140. Theimage-contrast-enhancing electrode 143 attracts the SE beams 102_1 se,102_2 se and 102_3 se generated by the probe spots 102_1 s, 102_2 s and102_3 s from the being-observed surface 7 and the particles thereon.Consequently, only the dark-field BSE beams 102_1 d 2, 102_2 d 2 and102_3 d 2 can travel upwards along the primary optical axis 220_1 andtherefore can be detected by the dark-field detection elements140_1˜140_3.

FIGS. 8A-8C take the beamlet 102_1 as an example to show how the imagecontrast is enhanced when there is one particle 7_P on thebeing-observed surface 7. In FIG. 8A, the beamlet 102_1 impinges on thesurface 7, the bright-field SE beam 102_1 b 1 is attracted by theimage-contrast-enhancing electrode 143 and consequently lands thereon,and the bright-field BSE beam 102_1 b 2 travels in the reflectiondirection of the surface 7. In FIG. 8B, the beamlet 102_1 impinges onthe particle 7_P, the dark-field SE beam 102_1 d 1 is attracted by theimage-contrast-enhancing electrode 143 and consequently lands thereon,and the dark-field BSE beam 102_1 d 2 travels in the reflectiondirection of the particle 7_P. Hence the SE beam 102_1 se in FIG. 7 isactually formed by either the bright-field SE beam 102_1 b 1 or thedark-field SE beam 102_1 d 1. FIG. 8C shows the signal 140_1 s of thedark-field detection element 140_1. The dash line and the solid linerespectively express the signals without and with, theimage-contrast-enhancing electrode 143. As the beamlet 102_1 approachesthe particle 7_P, the signal 140_1 s is getting stronger from the lowervalue 140_1 s_0 to the higher value 140_1 s_1. Therefore the particle7_P will be shown in the image with a darker background and a highercontrast when the image-contrast-enhancing electrode 143 is used.

Another embodiment 300A of the new multi-beam apparatus is shown in FIG.9, which detects the bright-field signal electron beams 102_1 b, 102_2 band 102_3 b generated by the probes 102_1S, 102_2 s and 102_3 s from thebeing-observed surface 7 of the sample 8. In comparison with theembodiment 210A in FIG. 3, one bright-field secondary projection imagingsystem 250 and one bright-field electron detection device 240 with threebright-field detection elements 240_1, 240_2 and 240_3 are placed alongthe bright-field secondary optical axis 250_1. For the surface 7, theprimary optical axis 300 is in the incidence direction and thebright-field secondary optical axis 250_1 is in the reflectiondirection. The bright-field signal electron beams 102_1 b˜102_3 b arefocused onto the bright-field detection elements 240_1˜240_3respectively by the bright-field secondary projection imaging system250, and therefore each bright-field detection element generates onebright-field image. One bright-field image comprises a material contrastand a topography contrast of the sample surface 7.

Similar to the foregoing dark-field secondary projection imaging system150, the bright-field secondary projection imaging system 250 maycomprise one anti-scanning deflector. The positions of the focusedbright-field signal electron beams 102_1 b, 102_2 b and 102_3 b on thebright-field electron detection device 240 will change as the probespots 102_1 s˜102_3 s scan the surface 7, and the anti-scanningdeflector can cancel the displacements of the positions and thereforeavoid the cross-talks. In addition, the bright-field secondaryprojection imaging system 250 can be configured to attract thebright-field SEs as much as possible to increase the signal-noise ratiosof the bright-field images. If the bright-field electron detectiondevice 240 is perpendicular to the bright-field secondary optical axis250_1, the slant surface 7 will defocus the bright-field signal electronbeams 102_1 b˜102_3 b, and therefore enlarge the sizes thereof. Thedefocusing may make a part of one focused bright-field signal electronbeam out of the corresponding bright-field detection element and therebygenerating the cross-talks. Hence the bright-field electron detectiondevice 240 can be tilted to compensate the defocusing due to the slantsurface 7.

Based on the embodiment 210A in FIG. 3 and the embodiment 300A in FIG.9, another embodiment 400A of the new multi-beam apparatus is proposed.FIG. 10 shows the configuration of the embodiment 400A and one of itsoperation modes. For the being-observed surface 7 of the sample 8, theembodiment 400A employs the dark-field secondary projection imagingsystem 150 and the dark-field electron detection device 140 to obtainthe dark-field images, and bright-field secondary projection imagingsystem 250 and the bright-field electron detection device 240 to obtainthe bright-field images.

As well known, the more beamlets scan the being-observed surface 7, themore charges may be built thereon. Hence for a specific observationapplication, some beamlets are better to be blanked, such as every otherbeamlet. In this case, those beamlets can be directed to be blanked bythe beamlet-limit plate 121 of the new source-conversion unit in theforegoing embodiments. FIG. 11A shows such an operation mode of theembodiment 210A in FIG. 3, wherein themicro-deflector-lens-and-compensator 122_1 dc deflects the beamlet 102_1to be blanked by the beamlet-limit plate 121. Any of all the foregoingembodiments can operate either one multi-beam mode or one single-beammode. One single-beam mode may be used when searching optimistic imagingconditions (landing energy and probe current) for an observationapplication.

To make the single-beam mode easy to use, the foregoing embodiments canfurther comprise one single-beam detector individually. Taking theembodiment 210A in FIG. 3 as an example, FIG. 11B shows anotherembodiment 211A of the new multi-beam apparatus, wherein a single-beamelectron detector 141 is added. Here the beamlet 102_1 is taken as thebeamlet in use. The beam separator 160 deflects the correspondingdark-field signal electron beam 102_1 d to be detected by thesingle-beam electron detector 141. Using the single-beam electrondetector 141 can avoid the procedures of adjusting the dark-fieldsecondary projection imaging system 150 with respect to the focusingpower variation of the objective lens 131. As mentioned above, thefocusing power of the objective lens 131 will change when the landingenergy and/or current of the beamlet in use are changed. Generallyspeaking, the single-beam detector 14 can collect more dark-field signalelectrons than one detection element of the dark-field electrondetection device 140, and therefore can provide a higher signal-noiseratio.

In summary this invention proposes a new multi-beam apparatus forobserving the being-observed surface of a sample with obliqueillumination. The apparatus can function as a yield management tool todefects and/or particles on wafers/masks with high detection sensitivityand high throughput in semiconductor manufacturing industry. The samplesurface is slant to the primary optical axis of the apparatus. Theapparatus employs a new source-conversion unit to form a slant virtualmulti-source array with a plurality of beamlets from a single electronsource, a primary projection imaging system to project the multi-sourcearray onto the slant sample surface and therefore form a plurality ofprobe spots thereon with oblique illuminations of the plurality ofbeamlets. The apparatus can use a condenser lens to adjust the currentsof the plurality of probe spots. In the new source-conversion unit, theimage-forming means is on the upstream of the beamlet-limit means, theimage-forming means comprises a plurality of micro-deflector-lenselements for forming the virtual multi-source array slant to the primaryoptical axis, or a plurality of micro-deflector-lens-and-compensatorelements for forming the slant virtual multi-source array andcompensating the aberrations of the plurality of probe spots.

The apparatus can use a beam separator to separate a plurality ofdark-field signal electron beams from the plurality of beamlets, and adark-field secondary projection imaging system and a dark-field electrondetection device to focus and detect the plurality of dark-field signalelectron beams and therefore obtain a plurality of dark-field images,wherein an image-contrast-enhancing electrode can be employed toincrease their image contrasts. The apparatus can use a bright-fieldsecondary projection imaging system and a bright-field electrondetection device to focus and detect the plurality of bright-fieldsignal electron beams and therefore obtain a plurality of bright-fieldimages. The apparatus can operate in a multi-beam mode or a single-beammode, and a single-beam detector can be added to make the single-beammode easy to use.

Although the present invention has been explained in relation to itspreferred embodiment, it is to be understood that other modificationsand variation can be made without departing the spirit and scope of theinvention as hereafter claimed.

What is claimed is:
 1. A charged-particle beam apparatus, comprising: acharged particle source configured to provide a primary beam; an imageforming unit configured to form a plurality of virtual images of thecharged particle source using a plurality of beamlets derived from theprimary beam, wherein the plurality of virtual images of the chargedparticle source are slant to a primary optical axis; a first projectionimaging system configured to form a plurality of probe spots on a samplefrom the plurality of beamlets, wherein the first projection imaginingsystem comprises an objective lens; a second projection systemconfigured to focus a plurality of secondary beams generated by theplurality of probe spots on the sample; and a detection device with aplurality of detection elements configured to receive the plurality ofsecondary beams.
 2. The charged-particle beam apparatus according toclaim 1, wherein the first projection imaging system includes a transferlens configured to focus the plurality of beamlets and enable theplurality of beamlets to land on the sample with equal amounts ofincidence angles.
 3. The charged-particle beam apparatus according toclaim 1, further comprising a beam separator configured to separate theplurality of beamlets and the plurality of secondary beams.
 4. Thecharged-particle beam apparatus according to claim 3, wherein theapparatus can operate in a single-beam mode.
 5. The charged-particlebeam apparatus according to claim 4, further comprising a single-beamelectron detector above the beam separator, which can be used in thesingle-beam mode.
 6. The charged-particle beam apparatus according toclaim 1, wherein the image forming unit comprises a plurality ofimage-forming elements.
 7. The charged-particle beam apparatus accordingto claim 6, wherein the plurality of image-forming elements comprises amicro-stigmator for compensating astigmatism aberration of onecorresponding probe spot.
 8. The charged-particle beam apparatusaccording to claim 6, wherein the plurality of image-forming elementsare configured to deflect the plurality of beamlets to changecorresponding incidence angles.
 9. The charged-particle beam apparatusaccording to claim 6, wherein the plurality of image-forming elementscomprises a 4-pole structure configured to function as a micro-deflectorand a micro-round-lens.
 10. The charged-particle beam apparatusaccording to claim 6, wherein the plurality of image-forming elementscomprises an 8-pole structure configured to function as amicro-deflector, a micro-round-lens, and a micro-stigmator.
 11. Thecharged-particle beam apparatus according to claim 6, wherein theplurality of image-forming elements comprises an upper 4-pole structureand a lower 4-pole structure thereunder, and the upper 4-pole structureand the lower 4-pole structure are aligned with each other and have a45° difference in azimuth.
 12. The charged-particle beam apparatusaccording to claim 11, wherein the upper 4-pole structure and the lower4-pole structure are configured to function as a micro-deflector, amicro-round-lens, and a micro-stigmator.
 13. The charged-particle beamapparatus according to claim 1, wherein the charged particle source, theimaging forming unit, and the first projection imaging system arealigned with the primary optical axis of the apparatus.
 14. Thecharged-particle beam apparatus according to claim 1, further comprisinga beam-limiting element for limiting the plurality of beamlets with aplurality of beam-limit openings.
 15. The charged-particle beamapparatus according to claim 14, further comprising a condenser lensconfigured to focus the primary beam to vary electric currents of theplurality of probe spots on the sample.
 16. The charged-particle beamapparatus according to claim 6, wherein the plurality of image-formingelements is configured to deflect the plurality of beamlets derived fromthe primary electron beam to form the plurality of virtual images of thecharged particle source.
 17. The charged-particle beam apparatusaccording to claim 6, wherein the plurality of image-forming elementsmay comprise a plurality of micro-round-lenses.
 18. The charged-particlebeam apparatus according to claim 17, wherein the plurality ofmicro-round-lenses of the plurality of image-forming elements focusesthe plurality of beamlets to tilt the virtual images of the chargedparticle source slant to the primary optical axis.